Lake Winnipeg (French: Lac Winnipeg) is a large, 24,514-square-kilometre (9,465 sq mi) lake in central North America, in the province of Manitoba, Canada, with its southern tip about 55 kilometres (34 mi) north of the city of Winnipeg. It is the largest lake within southern Canada's borders, and it is part of the most undeveloped large watershed of southern Canada.

Lake Winnipeg is Canada's sixth-largest freshwater lake,[3] and the third-largest freshwater lake contained entirely within Canada, but it is relatively shallow (mean depth of 12 m (39 ft))[4] excluding a narrow 36 m (118 ft) deep channel between the northern and southern basins. It is the eleventh-largest freshwater lake on Earth. The lake's east side has pristine boreal forests and rivers that are being promoted as a potential United Nations World Heritage Park. The lake is elongated and is 416 km (258 mi) from north to south, with remote sandy beaches, large limestone cliffs, and many bat caves in some areas. Manitoba Hydro uses the lake as one of the largest reservoirs in the world. There are many islands, most of them undeveloped.

The lake's watershed measures about 982,900 square kilometres (379,500 sq mi),[5] and covers much of Alberta, Saskatchewan, Manitoba, northwestern Ontario, Minnesota, and North Dakota. Its drainage is about 40 times larger than its surface, a ratio bigger than any other large lake in the world. Given the massive watershed and the relatively small volume of water in the lake, it is dominated by events in its watershed. It is not surprising to find it showing the effects of materials being added to it as a result of activities in the watershed.

Lake Winnipeg is suffering from many environmental issues such as an explosion in the population of algae, caused by excessive amounts of phosphorus seeping into the lake, therefore not absorbing enough nitrogen.[8][9] The phosphorus levels are approaching a point that could be dangerous for human health.[10]

Lake Winnipeg and Lake Manitoba are remnants of prehistoric Glacial Lake Agassiz, although there is evidence of a desiccated south basin of Lake Winnipeg approximately 4000 years ago. The area between the lakes is called the Interlake Region, and the whole region is called the Manitoba Lowlands.

It is believed Henry Kelsey was the first European to see the lake, in 1690. He adopted the Cree language name for the lake: wīnipēk (ᐐᓂᐯᐠ), meaning "muddy waters". La Vérendrye referred to the lake as Ouinipigon when he built the first forts in the area in the 1730s. Later, the Red River Colony to its south took the lake's name for Winnipeg, the capital of Manitoba.

Because of its length, the Lake Winnipeg water system and the lake was an important transportation route in the province before the railways reached Manitoba. It continued to be a major transportation route even after the railways reached the province. In addition to aboriginal canoes and York boats, several steamboats plied the lake, including Anson Northup, City of Selkirk, Colvile, Keenora, Premier, Princess, Winnitoba, Wolverine and most recently the diesel powered MS Lord Selkirk II passenger cruise ship.

Lake Winnipeg has important commercial fisheries. Its catch makes up a major part of Manitoba's $30 million-a-year fishing industry.[13] The lake was once the main source of goldeye in Canada, which is why the fish is sometimes called Winnipeg goldeye. Common carp were introduced to the lake through the Red River of the North and are firmly established. Walleye and whitefish together account for over 90 percent of its commercial fishing.[14]